In general, an electronically controlled suspension (ECS) apparatus is installed between a wheel axle and a vehicle body, so that it absorbs road shocks exerted on the vehicle axle to protect goods from damage and improve ride comfort. Further, the ECS apparatus controls an attitude (e.g. a roll, a squat or a dive) of the vehicle body experiencing forces incurred by, e.g., a turning, an acceleration, or a deceleration of a vehicle.
In order to perform such a vehicle attitude control, the suspension apparatus adjusts a damping force of a variable damper incorporated therein by activating an actuator of, e.g., a step motor type or a solenoid type, of the variable damper depending on driving conditions, such as a speed, a deceleration and an acceleration of the vehicle, and on a road surface condition.
For example, the variable damper having the actuator of the step motor type is provided with a control rod, which is rotated clockwise or counterclockwise by a certain angle by the actuator. And an effective cross-sectional area of an orifice in the variable damper is varied by the rotation of the control rod, which results in a variation of the damping force of the variable damper.
The variable dampers are usually classified into two types. One type is a reverse type, in which a rebound damping force and a compression damping force are individually controlled. The other is a normal type, in which the rebound damping force and the compression damping force are increased or decreased together.
By adjusting damping forces, the reverse type damper selectively produces three damper settings, i.e., a firm (or hard) rebound with a soft compression (hereinafter referred to as an H/S), a soft rebound with a soft compression (hereinafter referred to as an S/S) and a soft rebound with a firm (or hard) compression (hereinafter referred to as an S/H). And the normal type damper selectively produces two damper settings, i.e., a firm rebound with a firm compression (and hereinafter referred to as an H/H) and S/S.
Referring to FIG. 1, there is illustrated a conventional ECS apparatus for an anti-squat control, which includes a throttle position sensor (TPS) 10 for producing an electrical signal which is representative of a throttle opening angle, an electronic control unit (ECU) 20, a front left and front right dampers 30F and a rear left and rear right dampers 30R.
The conventional ECS apparatus performs an anti-squat control as follows: First, when the ECU 20 determines that the TPS signal produced from the TPS 10 becomes greater than a predetermined threshold ((a) of FIG. 2), it varies damping forces of the front and rear dampers 30F, 30R by activating actuators ((b) and (c) of FIG. 2). For example, if the TPS signal is greater than the threshold and the front and rear dampers 30F, 30R are of the reverse type, the ECU 20 sets the front variable dampers 30F into the H/S mode to provide hard rebound and soft compression strokes and sets the rear variable dampers 30R into the S/H mode to provide soft rebound and hard compression strokes. In contrast, if the front and rear dampers 30F, 30R are of the normal type, the ECU 20 sets the front and rear dampers 30F, 30R into the H/H mode to provide hard rebound and hard compression strokes.
The ECU 20 increases or decreases the damping force of the damper by a fixed amount, or by an amount proportional to a level of the TPS signal or of a differentiated value thereof. Further, the dampers are set into the S/S mode again after a predetermined period of time, i.e., a control time T1, so that the riding comfort can be recovered.
In such a conventional ECS apparatus, a squat (or nose-up) occurring when the TPS signal increases can be suppressed; however, when an acceleration pedal is released so that the TPS signal decreases and the vehicle is not accelerated anymore (see (d) of FIG. 2), there still occurs a dive, i.e., a sudden drop of a nose portion of the vehicle, thereby resulting in deterioration of the riding comfort of the vehicle. (In FIG. 2, it is assumed for simplicity that the vehicle is being accelerated from time t0 to t1.)